The right middle lobe is bordered superiorly by the horizontal fissure, and medially by the right heart border. Any abnormality, which increases density of this lobe, may therefore obscure the right heart border, or be limited superiorly by the horizontal fissure.
This x-ray of Child with a cough and fever shows right middle lobe consolidation :
* Consolidation is limited above by a crisp line, formed by the horizontal fissure "red arrow"
* The pathology must therefore involve the right middle lobe
* More extensive shadowing also involves the right and left peri-hilar regions
* Pneumonia involving the right middle lobe